1. Technical Field
In general, the invention relates to a sensor preferably comprising a multi-layered ceramic substrate with at least one sensor element being arranged and/or embodied in, at, or on the ceramic substrate. Further, the invention relates to a method for the production of such a sensor.
While electronic assemblies have previously been produced on conventional circuit boards, today they are increasingly generated on and/or in ceramic substrates. The use of ceramics shows several advantages at once. Here, the high temperature resistance of ceramics is of particular importance. Additionally, the expansion of ceramics is considerably lower over the temperature progression than circuit boards of the prior art. Additionally, it is possible to produce multi-layered ceramics with integrated conductors in and/or between said layers. It is even possible to implement electric/electronic components between these layers. This leads to an extremely compact design. The contacting of such electronic assemblies commonly occurs via so-called soldering pads printed onto the substrate. In soft soldering methods of the prior art the contact is soldered onto the soldering pads in the form of a strand, metal wire, or the like. Soft welding is disadvantageous in that the soldering tin used already melts at temperatures from 200° C. to 300° C. This eliminates the enormous advantage of the high temperature resistance of ceramics, which cannot be utilized when the conventional soldering tin is used for soft soldering.
2. Description of Related Art
Solderings of the prior art are also disadvantageous, since in the connection to the solder pad is an exclusively electric connection. Such a connection is extremely unstable with regards to mechanic stress. Accordingly, the soldering site may not be subjected to any considerable mechanic stress. In case of shock, vibration, or other mechanic stress such soldering sites frequently break or tear. Consequently a separate mechanic fixation is required, for example by adhesion, joining, etc.
Hard soldering methods are an alternative to conventional soft soldering, with here the soldering temperature exceeding 450° C. Active brazing is a particular form of hard soldering. Commercial active brazing shows a melting point of approx. 850° C. Additionally, it shows the advantage that via active brazing metal the brazing process can occur directly on the ceramic. The conventional metallization of the ceramics required in conventional soft soldering or hard soldering can be omitted for active brazing, because the solder enters into a direct connection with the surface of the ceramics due to its chemical composition. By adding specific soldering elements, such as titanium, a reaction develops in active brazing at the boundary, namely between the metal and/or the metal alloy of the solder and the surface of the ceramic, so that the materials directly connect to each other without any adhesive agents being interposed. The particular advantage of active brazing is therefore given in the high mechanic resistance and high temperature stability of the connection. Consequently, a metal-ceramics connection suitable for high temperatures can be produced by active brazing.
Sensors of a generic type are known in practice. Such sensors already exist comprising multi-layered ceramics. For example, reference is made here to DE 10 2008 016 829 A1 and DE 103 14 875 A1.
In the sensors in question the connection between metal and ceramics represents a particular problem, too. Due to the fact that the actual sensor element is usually arranged on or in the ceramic and/or the ceramic substrate and for example at least one measuring electrode or at least one measuring coil must be contacted electrically, in general there is the problem how such an electric connection can be generated flawlessly and lastingly while simultaneously generate sufficiently good mechanic connections.
Usually, metallic contacts for sensors are also applied on ceramics, for example by way of soldering. Per se, such metal-ceramic soldering connections are also known from the prior art. Here, reference is made to DE 10 2004 024 920 B4. Concretely, from DE 10 2004 024 920 B4 a pressure sensor is known, in which a ceramic substrate is connected via active or hard soldering to a support structure made from metal, connected fixed to the connection housing of the pressure sensor. Here, it is disadvantageous that the ceramic substrate only serves as a fastening device for the actual pressure measuring cell. This pressure measuring cell in turn is made from metal and connected to the ceramic substrate by way of active brazing on one side. On the other side the ceramic substrate is provided with connection pins.
The measures of the prior art for electric contacting sensors are problematic in practice, however, since on the one hand they cause considerable expenses in handling/production and on the other hand they are only conditionally resistant to external-mechanical-influences. Such contacts generated by soldering are extremely “sensitive” and thus subject to malfunction.